The invention relates to a method of switching the electric supply between two independent load circuits, applicable in particular to X-ray tubes utilised in modern radiology.
The radiological applications most widely known, particularly in the medical sphere, are radioscopy and radiography. An object or a patient is exposed to the radiation of an X-ray tube in these two applications, and the image of the attenutation of the beam of rays after traversal through the object is picked up. The X-rays pass through the object for protracted periods in radioscopy. In the medical sphere, the intensity of this radiation is low in such a manner as not to expose the patient to a protracted harmful irradiation. In radiography, the intensity of the X-rays is high but its period is short. This procedure has the result of effectively exposing the photographic plate which records the image whilst refraining from subjecting the object or the patient to an excessive irradiation due to the briefness of this single pulse.
The X-ray tubes able to radiate one of these two radiations as desired have a single complete structure as a common body, in particular comprising an anode, focussing devices, control devices, and a mechanical tube envelope. Switching circuits act on a cathode array of this tube. A selection is made in this manner, as needed, of a cathode emitting few or numerous electrons, such that an emission of low or high intensity X-rays respectively is produced when these electrons strike the anode. The control device of the tube commonly comprises means such that the cathode polarisation may be cancelled rapidly in a radiography application.
In practice, the cathode array of the tube comprises two cathodes. These cathodes currently comprise filaments heated up by direct heating by passing an electric heating current through these filaments. For the purpose of limiting the electrical connectons which must pass through the tube envelope, it is known that these filaments may be connected to a common point which also serves the purpose of applying the polarising voltage to the cathode. This cathode array consequently comprises at least three connecting terminals. In this way, the polarising or biasing voltage is fed simultaneously to both the cathodes of the cathode array, but only that one of these cathodes which is exposed to a heating current is in operation. The known switching devices commonly comprise an electromechanical relay providing a choice, or else two different transformers, the switching operation then being performed at the level of the primary windings.
The insulation of this relay is difficult, since the cathodes are biased with respect to the earth of the apparatus by a voltage close to several tens of thousands of volts. The disruption problems which may arise between its armature and its terminals are difficult to resolve. Their lack of insulation raises a serious risk. Furthermore, generators of different electrical signals are frequently utilised to provide the heating of each of the two cathodes.